Surface water balances at node 21 (Figure 3(c) of Section 2.5.1) in the Mammy Johnsons River are provided in Table 5 for 2013 to 2042, 2043 to 2072 and 2073 to 2102. Groundwater balances for the alluvial groundwater in the Karuah river basin (Figure 3 of Section 2.5.1) are provided in Table 6 for 2013 to 2042, 2043 to 2072 and 2073 to 2102. The median changes in the water balance terms attributable to the additional coal resource development are illustrated in Figure 5 for surface water and Figure 6 for groundwater. The water balances are summarised separately because, as stated in Section 2.5.1, they apply to different areas. The surface water balance applies to an area of 308.6 km2, whereas the alluvial groundwater model extent is 26 km2. Rainfall inputs over the three reporting periods reflect changes due to a varying climate signal.
Licensed extractions for non-mining uses are not reported because they were not represented in the model (see Section 126.96.36.199). The impact of licensed extractions of groundwater for CSG and dewatering of coal mines are reflected in the upward flow from deeper groundwater water balance term.
Within each table, the impact of the ACRD, calculated as the difference between the baseline and the CRDP, reflects the impacts of mining and CSG development only, as the climate signal is identical in both modelling runs.
The differences between corresponding values over the three reporting periods encompass both the effects of climate and the coal resource development.
The effects of climate change on the water balance in the Mammy Johnsons River are very small. There is a decreasing trend in rainfall which flows through to a decreasing trend in the surface water outflow and groundwater recharge. However, the magnitude of the change is very small, with less than 1% decrease in rainfall, less than 2% decrease in surface water outflow and less than 3% decrease in groundwater recharge when the 2073 to 2102 period is compared to the 2013 to 2042 period for the baseline.
The reduction in the median surface water outflow as a consequence of the additional coal resource development is on average greatest (186 ML/year) between 2043 and 2072, and remains at this level through to the end of the simulation in 2102 (Table 5). The majority of this impact is attributed to the retention of surface runoff on-site rather than groundwater extraction because the decrease in baseflow is negligible compared to the decrease in surface water outflow for all three periods of time (Table 6). The surface water impact is sustained over the modelling period at the open-cut Duralie Coal Mine because most of the surface water footprint is due to interception of runoff by small dams, which are assumed to remain in place following the cessation of mining (see Figure 7 in companion product 2.6.1 for the Gloucester subregion (Zhang et al., 2018)). The decrease in surface water outflow is reflected in an increase in the residual term. Most of the water lost is from evapotranspiration, although leakage of water from open-cut pits and mine water storages may account for a small component and there will be a small change in catchment storage. The decrease in the CRDP surface water outflow is 0.3% of the baseline surface water outflow and within the uncertainty of the predicted surface water outflow.
As discussed in Section 188.8.131.52, some modelling artefacts are evident in the water balance results. In the Mammy Johnsons river basin, the CRDP recharge in the period 2073 to 2102 is greater than the baseline recharge (Table 6), which indicates a smaller drawdown under the CRDP, hence fewer surface cells have dried out relative to the baseline. In addition, the location of groundwater pumping under the CRDP has shifted from that for the baseline and this has introduced another modelling artefact, which results in more baseflow under the CRDP than baseline. Thus the increase in annual baseflow under the CRDP in the third period is the net effect of the additional coal resource development and a higher recharge relative to the baseline. Overall, there are negligible changes in the groundwater components of the water balance between the baseline and CRDP for the time periods 2013 to 2042 and 2043 to 2072 (Table 6), and these small numerical modelling issues are not materially impacting the results.
Table 5 Mean annual surface water balance at node 21 on Mammy Johnsons River for 2013 to 2042, 2043 to 2072 and 2073 to 2102 in the Gloucester subregion
For some (but not all) terms, three numbers are provided. The first number is the median, and the 10th and 90th percentile numbers follow in brackets. NM = data not modelled, ET = evapotranspiration
Table 6 Mean annual groundwater balance for the alluvial groundwater model extent in the Karuah river basin for 2013 to 2042, 2043 to 2072 and 2073 to 2102 in the Gloucester subregion
The first number is the median, and the 10th and 90th percentile numbers follow in brackets.
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- 2.5.1 Methods
- 2.5.2 Water balances
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